1. Field of the Invention
The present invention relates to an organic EL pixel circuit for controlling application of a drive voltage to an organic EL pixel.
2. Description of Related Art
Use of organic EL panels as flat panel displays has been conventionally known. Because the pixels in an organic EL panel are self-illuminating, an organic EL panel has advantages including that, unlike a liquid crystal display, no backlight is required and that the display is relatively bright.
FIG. 8 illustrates an example structure of a pixel circuit in an organic EL panel employing conventional thin film transistors (TFTs). An organic EL panel is composed of these pixels arranged in a matrix.
FIG. 8 shows the gate of a selection transistor TFT1, which is an n-channel thin film transistor to be selected by a gate line and which will hereinafter be referred to simply as TFT1, connected to a gate line extending in the row direction. The drain of the TFT1 is connected with a data line extending in the column direction. The source of the TFT1 is connected with one end of a storage capacitor SC having the other end connected a storage capacitor power source line. The node connecting the source of the TFT1 and the storage capacitor SC is connected with the gate of a drive transistor TFT2 which is a p-channel thin film transistor (which will be hereinafter referred to simply as TFT2). The source of the TFT2 is connected to a power source PVDD and the drains of the TFT2 is connected with one end of an organic EL element EL. The other end of the organic EL element EL is connected with a cathode power source CV.
In the circuit thus configured, when the gate line is at H level, the TFT1 is turned on, and the data in the data line at this point is stored in the storage capacitor SC. The TFT2 is switched on and off in accordance with the data (potential) held by the storage capacitor SC. When the TFT2 is on, an electrical current flows through the organic EL element EL, which then emits light.
Light emission of each pixel is controlled in the manner described above. Because of the existence of the storage capacitor SC, the organic EL element EL is capable of emitting light even after the TFT1 is turned off. The storage capacitor SC typically retains the ON or OFF state of the TFT2 until the next gate line is selected.
In an organic EL panel employing such above-described TFTS, the pixels arranged in a matrix and each including the organic EL element, TFT1 and TFT2, are disposed on the same substrate. This structure results in generation of a parasitic capacitor in the organic EL element EL.
Accordingly, such a conventional pixel circuit has a problem that even when the TFT2 is off, a current flows in the organic EL element EL in accordance with the charges accumulated in the capacitor of the organic EL element, thereby generating an afterimage. More specifically, while the panel operates at a high response speed when the organic EL element is turned on, the response becomes slower due to the influence of the capacitor of the organic EL element when the organic EL element is turned off, with a result that afterimages are common.